The present invention relates generally to a magnetic resonance (MR) imaging and, more particularly, to a system and method for identifying periodic motion complexes for MR imaging triggering.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Magnetic resonance imaging is a diagnostic imaging technique commonly used to review, identify, and diagnose pathologies or abnormalities in a scan subject, e.g., medical patient. For example, MR images of the cardiac region are often used by health care professionals to diagnose medical conditions. Traditional MR evaluations of the cardiac region often rely on repeated cardiac-gated acquisition of MR data in order to reduce image degradation resulting from the continuous movement of the cardiac region due to respiratory and/or circulatory physiological functions.
To achieve a cardiac-gated acquisition of MR data, systems have been developed that rely on detection of a particular point in the motion cycle as a trigger to repeatedly acquire data at approximately the same phase of the motion cycle. An electrocardiogram (ECG) is generally utilized to monitor the cardiac cycle and identify a particular peak, often an R-peak, within the ECG waveform. By identifying an occurrence of the R-peak, these systems infer that a Q-peak and S-peak are associated with the R-peak and thereby identify an occurrence of a QRS complex. The identification of a QRS complex is then used as a point for triggering the acquisition of MR data from the subject to be imaged.
However, the R-peak can often be distorted or obscured by strong noise associated with MR environments. For example, spikes may be induced within the ECG waveform by RF pulses or gradient pulses and may be misidentified as an R-peak. Accordingly, such systems may infer the spike as an R-pulse of a QRS complex and mistrigger imaging. Furthermore, abnormal patient conditions such as premature ventricular contraction (PVC) may hinder detection of an R-peak. Accordingly, many identification systems may cause imaging to be triggered prematurely or fail to trigger entirely thereby degrading the image quality and extending scan durations.
Accordingly, systems have been designed to review multiple features of the ECG feedback or Vectorcardiogram (VCG) feedback to provide an accurate detection of an R-peak. Specifically, some systems monitor an amplitude and a first derivative of the ECG waveform and assume that the R-peak will be the highest magnitude in the ECG waveform. Within these systems, the assumption that the R-peak will have the largest amplitude provides the fundamental basis for detection of the R-peak. However, this assumption also provides a primary basis for inaccuracies in detecting the R-peak and may lead to mistriggering or trigger lapses during gated MR processes.
That is, systems relying on amplitude and first derivative of the ECG waveform for detection of an R-peak may misinterpret artifacts or noise that have an amplitude larger than a given R-peak as an R-peak. Specifically, when a spike, T-Swell, or PVC are present, which have similar amplitude to the amplitude of a QRS complex, such amplitude-based systems may fail to identify R-peaks or erroneously identify R-peaks. For example, magnetohydrodyamic spikes may have a larger amplitude than an R-peak. These spikes may cause an amplitude-based triggering system to detect the spike as an R-peak and mistrigger data acquisition. Additionally, noise associated with the MR imaging process may cause the first derivative of the ECG waveform to be higher than that of the QRS complex and cause a mistrigger or trigger failure. As a result, both amplitude and first derivative based algorithms may result in poor image quality and require re-scanning of the subject.
It would therefore be desirable to have a system and method capable of accurately detecting a cyclical physiological point to trigger imaging in the presence of noise and artifacts. Specifically, it would be desirable to have a robust, reliable, and complete system for evaluating information within ECG or VCG waveforms to improve accuracy of triggered data acquisitions.